1. Field of the Invention
The present invention is broadly concerned with a method of fabricating complex superconducting oxides through appropriate selection and mixing together of a new fundamental substructure or "building block", which is intrinsically superconducting, together with stoichiometric proportions of the oxides making up another intrinsically superconducting fundamental structure, whereby a virtual infinity of complex oxides can be formed. Thus, the invention comprehends a completely rationalized method of fabricating complex oxides of desirable T.sub.c values using the fundamental substructures. In another aspect of the invention, stable, superconducting 112 and 12 ceramic oxides are described.
2. Description of the Prior Art
Superconductivity refers to that special state of a material where its resistance to electrical current flow suddenly and completely disappears when its temperature is lowered. Below this onset or critical temperature T.sub.c, a characteristic of the material, the electrical resistance does not merely drop to a low level but it vanishes entirely. Only a very limited list of materials exhibit such a state. The discovery of the first superconductor occurred in 1911. Heike Kammerlingh Onnes discovered that Mercury lost all detectable resistance at a temperature just 4.degree. above absolute zero.
A superconductor also exhibits perfect diamagnetism below its critical temperature, i.e., it expels all magnetic field lines from its interior by producing an opposing magnetic field from a current flowing on its surface. As a consequence of the perfect diamagnetism of superconductors, they can be used to produce magnetic levitation as envisioned in high speed transport systems of the future, where magnetic repulsion is used to counter gravity. The perfect diamagnetism property of superconductors is called the Meissner effect after its discoverer.
Superconductivity is the only large scale quantum phenomenon involving charges found in solid materials. The current-carrying electrons in the superconductor behave as if they were part of a monumentally large single molecule the size of the entire specimen of the material. The macroscopic quantum nature of superconductors makes them useful in measuring magnetic field quantities to high precision or facilitates the measurement of such quantities so small as to be heretofore unmeasurable.
Hence, all three aspects of superconductors give promise of exciting new technologies or improvements in old technologies. However despite the tremendous potential of superconductors, formidable technical problems must be overcome if such materials are to achieve practical commercial application. For example, until very recently, all known superconducting materials attained their superconducting state only at very low (cryogenic) temperatures on the order of 4.degree.-20.degree. K. Such low temperatures had to be reached by evaporating liquid helium, the only substance that remains liquid down to temperatures approaching absolute zero. The few sources of helium in nature and its expensive processing make it a very costly cryogenic fluid.
In recent years, a plethora of new superconducting oxides have been announced by researchers around the world. While these new materials have relatively high critical temperatures on the order of 80.degree.-130.degree. K., they are plagued by a number of intractable problems. For example, certain of these prior materials, while they have high T.sub.c values, have very low (e.g., 1 ampere/cm.sup.2 current density values, particularly when the materials are bulk sintered and therefore have random crystal orientation therein. Such prior materials may exhibit higher current densities, but only when formed as oriented epitaxal films on substrates. Obviously, such materials, while they exhibit superconducting properties, are totally impractical for use in most commercial applications. Finally, many of these prior superconductors are extremely brittle and frangible, which again effectively precludes their use as commercial-scale electrical conductors for example.
Accordingly, while there is recent intense interest in superconducting materials, presently available oxides of this character have one or more serious deficiencies which render them useless in commercial applications.
A persistent problem in the art of superconductivity is the lack of a fundamental understanding of the underlying rationale of the oxides and how they can be fabricated. Therefore, researchers have been forced to pursue an essentially ad hoc program of development without an overarching theory to guide them. The situation is very analogous to the early study of genetics before DNA had been properly characterized, where researchers were literally "groping in the dark." After DNA was characterized, however, geneticists were able to more rationally study genetics and this led to a tremendous increase in activity in that science. Similarly, there is a need in the superconductor art for a fundamental breakthrough which would provide an overall explanation for superconductivity and a more rational approach to the development and fabrication of complex superconducting oxides.